1. Field of the Invention
The present invention relates to a photothermographic material, and in particular to a photothermographic material that gives a high-quality image with reduced development irregularity and is improved in image storability.
2. Description of the Related Art
Recently, there has been a strong demand for a reduction in the amount of waste liquid in the field of medical imaging from the viewpoints of environmental conservation and space saving. For this reason, there is a need for technology regarding photothermographic materials serving as films for medical diagnosis and for photographic applications which photothermographic materials can be efficiently exposed to light with a laser image setter or a laser imager and which can form a clear black image with high resolution and high sharpness. Such photothermographic materials do not require use of liquid processing chemicals and can provide users with a thermal development system which is simpler and which does not contaminate the environment.
While similar requirements also exist in the field of general image forming materials, images for medical imaging is required to have particularly high image quality excellent in sharpness and granularity. This is because those for medical imaging are required to be finely represented. Moreover, those for medical imaging are characterized in that images of blue-black tones are preferred from the viewpoint of easy diagnosis. At present, various kinds of hard copy systems utilizing dyes or pigments such as ink jet printers and electrophotographic systems have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Meanwhile, photothermographic systems utilizing organic silver salts are already known. In particular, photothermographic materials generally have an image-forming layer in which the catalytically active amount of a photocatalyst (for example, silver halide), a reducer, a reducible silver salt (for example, an organic silver salt), and optionally a tone adjusting agent for controlling the color tone of silver are dispersed in a binder. After imagewise exposure, a photothermographic material is heated to a high temperature (for example, 80° C. or higher) to cause a redox reaction between a silver halide or a reducible silver salt (functioning as an oxidizer) and a reducer, forming a black silver image. The redox reaction is accelerated by the catalytic action of a latent image on the silver halide generated by the imagewise exposure. As a result, the black silver image is formed in an exposed region of the material. Much literature describes such photothermographic materials. FUDI MEDICAL DRY IMAGER FM-DP L™ serves as a medical image-forming system which uses photothermographic materials, and is put into practical use.
Thermal development does not require processing solutions, which are needed for wet development, and allows simple and rapid processing. However, the thermal development has unsolved problems that are not found in wet development. One of these is instable development. That is, the thermal development is advantageous in that it can be conducted by heating alone, but is problematic in that development irregularity occurs due to even a slight fluctuation or irregularity of the temperature of the heating surface of a heater. Moreover, the photographic characteristics of photothermographic materials are vulnerable to fluctuation in the environmental conditions surrounding heat development devices such as temperature and humidity. The other problem is that raw storage storability of photothermographic materials and storage stability of images formed should be improved. The reason why these properties are not good is that all the ingredients needed for image formation are contained in the coated films of the materials and, even after image formation, the unreacted ingredients and the reacted products remain in the film.
Use of a heat solvent to improve developing stability is known (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2004-78137 and 2004-86123). Use of a polymer latex binder containing a particular amount of polymerization initiator, a high-purity organic argentate salt, and/or a donor compound to improve storage stability is known (see, for example, JP-A Nos. 2003-337393, 2003-131336, and 2003-233149).
However, even with these compounds, such photothermographic materials are still unsatisfactory and there exists a demand for further improvement.
That is, there exists a need for a photothermographic material that gives a high-quality image with reduced development irregularity and is improved in image storability.